Decoder matrix for quadraphonic sound systems

ABSTRACT

An apparatus for decoding four individual audio signals contained in four encoded composite signals to recover the four individual audio signals in substantially their original form. The apparatus minimizes the number of all-pass phase-shifting networks necessary to perform the decoding.

United States Patent Bauer Apr. 1, 1975 3.786193 [H974 Tsurushimn [79 1004 ST [75] Inventor: Benjamin B. Bauer. Stamford. Primary Examiner-Kathleen H. Clnff) Conn. Assismn/ Examiner-'lhomas D'Amico Assignee: Columbia Broadcasting System gang/11w Agent. or F/rmMnrt1n Nova/2k; Spcncer E.

New York, NY.

[22] Filed: Apr. 18, [974 211 Appl. No.: 462,043 [57] ABSTRACT An apparatus for decoding four individual audio sig 152] [3.8. CI 179/] GQ, [79/1004 5T mils contained in four encoded composite signals to {51] Int. Cl H04r 5/00 recover the four individual audi s gnals in substan- [58] Field of Search [79/1004 ST 1 6Q 15 BT tiaiiy their original form. The apparatus minimizes the number of all-pass phase-shifting networks necessary [56] References Ci d to perform the decoding.

UNITED STATES PATENTS 3.761.628 9/1973 Bauer 179/1 00 8 Clams 8 Drawmg figures 3// L 335 L 32/ f L f 1 +3 no f b L0! 37 JIR ,4 336 .m R 9,. A 32;? Rf

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PR/OR ART PRIOR ART R'JENIED 9 3.875.338

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DECODER MATRIX FOR QUADRAPHONIC SOUND SYSTEMS BACKGROUND OF THE INVENTION This invention relates to audio systems and. more particularly. to an improved decoder for use in a quadraphonic sound system.

In my U.S. Pat. No. 3.76l.628 there is disclosed a sound system wherein four individual audio signals. designated L L,,, R,, and R; are encoded in accordance with the SQ quadraphonic technique to produce two composite signals designated L and R and are also encoded to produce two additional conjugate" composite signals which may be designated L-,* and R The referenced patent demonstrates that L and R, can be decoded in conventional fashion using an SQ decoder matrix to produce four signals designated L;'. L,,', R,,' and R,'. each of these signals containing. in predominant proportion. a corresponding one of the four individual audio signals. along with certain unwanted components in sub-dominant proportions. For reproducing equipment that is capable of obtaining only L and R these four signals, L,'. L,,'. R,,' and R, suffice as satisfactory. although not fully discrete." outputs for audio reproduction. The patent demonstrates that L and R-,-*. when available. can also be processed using an S type of decoder. to produce four signals which may be designated L;* L,,'*. R,,* and R,'*. and these latter four signals can be added to L,', L,,'. R,.' and R respectively, to recover the original four individual audio signals in fully discrete form.

The pair of decoding matrices used for the full *dis cretizing operation, as set forth in the referenced patent. operates satisfactorily. but utilize a total of eight all pass phase shifting networks (four in each matrix decoder). Each of these phase shifting networks is relatively expensive to implement. and it is an object of the present invention to provide improved decoder matrices whereby the two composite signals. L and R and their conjugates, L and R can be decoded to recover four fully discrete audio signals.

SUMMARY OF THE INVENTION The present invention is directed to an apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form. A first of the composite signals contains the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, while a second of the composite signals contains the second individual audio signal in dominant proportion and the third and fourth audio signals in sub-dominant proportions and phase shifted with respect to each other. The third and fourth composite signals are respectfully conjugates of the first and second composite signals. In accordance with an embodiment ofthe invention there is provided a first combining means which receives the first composite signal and the third composite signal and generates first and second processed signals which are a function of the sum and difference of the first and second composite signals. A second combining means receives the second composite signal and the fourth composite signal and generates third and fourth processed signals which are a function of the sum and difference of the first and second composite signals. A first all-pass phase shifting circuit is employed for phase shifting the first processed signal to obtain a first decoded signal. A second all-pass phase shifting circuit is employed for phase shifting the second processed signal to obtain a second decoded signal. Further provided is a third all-pass phase shifting circuit for phase-shifting the third processed signal by two different relative phase angles to obtain first and second auxiliary signals. Similarly. a fourth all-pass phase shifting circuit is provided to phase shift the fourth processed signal by two different relative phase angles to obtain third and fourth auxiliary signals. The first and fourth auxiliary signals are then combined to obtain a third decoded signal while the second and third auxiliary signals are combined to obtain a fourth decoded signal. In this manner. only six all-pass phase shift networks need be employed to obtain the four individual audio signals in substantially their original form.

In accordance with additional embodiments of the invention. even fewer all-pass phase shift networks can be employed to recover the four individual audio sig nals in discrete form. In these embodiments the ultimately recovered outputs have relative phase orienta tions which are shifted somewhat from their original relative orientations. but from the standpoint of a listener the result may be considered acceptable as a trade-off against cost savings in the decoder apparatus.

Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 show prior art encoding matrices of a type useful for developing composite signals that can be decoded using the present invention;

FIGS. 3 and 4 show decoding matrices ofa type employed in the prior art;

FIG. 5 illustrates a further decoding concept employed in the prior art;

FIG. 6 is a block diagram of an embodiment of the decoder in accordance with the invention;

FIG. 7 shows a block diagram of another embodiment of the decoder in accordance with the invention; and

FIG. 8 shows a block diagram of still another embodiment of the decoder in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To facilitate understanding of the present invention it is helpful to review certain aspects of the SQ type quadraphonic system set forth in my above-referenced U.S. Pat. No. 3.76l,628. FIGS. 1 and 2 show encoding matrices of the type disclosed in the patent and having characteristics described in detail in the co-pending U.S. application Ser. No. 384.334 filed July 3]. I973. and assigned to the same assignee as the present appli cation. The encoder of FIG. 1 has four input terminals. l4, l6, l8 and 20 to which the four signals L,. L R and R,, are respectively applied. The full L, signal is added in a summing junction 22 to .71 of the R signal. the output of the summing junction being applied to a phase shifting network 24 which introduces a reference phase shift 11; that varies continuously with frequency. The full R, signal at terminal 20 is added in a second summing circuit 26 to .7I of the L,, signal appearing at input terminal 16, and the output is passed through a second d1 network 28 which also provides the reference phase shift 11!. The L,, and R signals are also applied to respective networks 30 and 32. each of which provides a phase shift of (1b 90) and wherein the lb functions are essentially the same. The full signal appearing at the output of network 24 is added in a summing circuit 34 to .7l of the signal appearing at the out put of the network 30 to produce at its output terminal 36 a composite signal designated L Similarly. the full signal from network 28 is added in summing junction 38 to -.7] ofthe signal from network 32 to produce at its output terminal 40 a composite signal designated R The composite signals L and R' are conveniently characterized by the phasor groups 42 and 44. respectively.

Referring to FIG. 2. the four input signals can be cn coded in a slightly different manner to obtain outputs which are conjugates" of the previously developed composite signals L-,- and R For purposes of this application. the conjugate of a given composite signal is defined as a signal which when added to or subtracted from the given composite signal yields a result which contains only the component that had been predominant in the given composite signal. In other words. the sub-dominant components of the given composite signal and its conjugate are proportioned and oriented so as to cancel. In FIG. 2 the full L, signal is added in a summing junction 46 to .7l of the R,. signal. the output of the summing junction being applied to a phase shifting network 48 which introduces a reference phase shift 11/. The full R, signal at terminal 20 is added in sum ming circuit 50 to .71 of the L,, signal at input terminal 16 and the output is passed through the network 52 which also provides the reference phase shift :11. The L,, and R signals are also applied to respective networks 54 and 56. each of which provides a phase shift of (1!; 90). The full signal appearing at the output of network 48 is added in a summing circuit 58 to .71 of the signal appearing at the output of network 54 to produce at its output terminal 60 a composite signal designated Lf". Similarly. the full signal from network 52 is added in summing junction 62 to .71 ofthe signal from network 56. to produce at its output terminal 64 a composite signal designated R As with the encoder of FIG. I, in the phasor groups 66 and 68, representing the composite signals L and R respectively. the L, and R, signals are in phase and each have unity value, the L and R,, signal components in one of the composite signals are in substantial phase quadrature with the corresponding signal components in the other composite signal. and the L,, and R,, signals at output terminal 60 lag and lead. respectively. the L and R signals at terminal 64. The R,, signal in phasor group 66 aligned with but out of phase with the L; signal. and in phasor group 68 the L,, signal is aligned with and in phase with the R; signal.

Having developed the four composite signals. L R LR and RH. it will be recognized that they will generally be applied to the appropriate channels of the medium (i.e.. typically recorded or broadcast), with L and R being applied to principal channels that are most easily recoverable and L and R being applied to supplementary channels that can be recovered using more sophisticated equipment.

FIGS. 3 and 4 show decoding matrices of the type disclosed in the above-referenced U.S. Pat. No.

3.76l.628 and having characteristics described in dctail in the copending US. application Ser. No. 338.69l. filed Mar. 7. 1973. and assigned to the same assignee as the present application. The decoder of FIG. 3 includes a pair of input terminals 70 and 72 to which the composite signals L and R, are respectively applied. The signal applied to terminal 70 is applied in parallel and phase shifted by a pair of ill networks 74 and 76. and the signal applied to input terminal 72 is applied in parallel to networks 78 and 80. These networks are of the type previously described. the networks 74 and introducing a reference phase shift of lb and the networks 76 and 78 introducing a phase shift ofhl; The output signals from networks 74 and 80 are respectively applied directly to the left-front" output terminal 82 and to the right-front output terminal 84. Equal portions of the outputs of networks 74 and 78 are summed in a summing junction 86, the output of which is applied to the right-back output ter minal 88 and equal portions of the outputs of networks 76 and 80 are inverted and added in a second summing network 90. the output of which is applied to the leftback" output terminal 92. As a result of the described phase shifting and summing action. four decoded sig nals designated L,'. L,,. R,,'. and R, appear at output terminals 82, 92. 88 and 84 respectively. and have the composition depicted by phasor diagrams 94, 96. 98 and 100. respectively. It is seen that the predominant components in the four decoded signals. namely L,. L R,, and R,. have the same relative amplitude and phase as the corresponding signals applied to the encoder of FIG. 1. but that the predominant front" components are accompanied by reduced amplitude components from the back pair of channels and the predominant back components are accompanied by reduced amplitude components from the front pair of channels.

The decoder of FIG. 4 likewise has a pair of input terminals 102 and 104 to which the composite signals L and RH are respectively applied. The L signal is applied in parallel to a pair of phase shifting networks 106 and 108 which respectively introduce a reference phase shift ml: and a phase shift of (ll: 90). and the R signal is applied in parallel to a pair of similar phase shift networks 110 and 112. As in the decoder of FIG. 3, the outputs of phase shift networks 106 and H2 are respectively applied directly to the left front" output terminal 114 and the right front" output terminal I16. Equal portions of the outputs of networks 108 and I 12 are summed in a summing junction 118, the output of which is applied to the left back" output terminal 120, and equal portions of the outputs of networks 106 and 110 are inverted and summed in a second summing network 122, the output of which is applied to the right back" output terminal 124. Again, the four decoded signals appearing at the output terminals designated L,'*, L,,'*. R,,* and R,'* and represented by phasor groups I26, 128, and 132, respectively contain predominant signal components L L R and R, each accompanied by reduced amplitude signals from the two other channels. Again. also. the predominant signals have the same relative amplitude and phase relationships that they exhibited at the input terminals of the encoder of FIG. 2.

Inspection of the two sets of phasor groups from the two encoders reveals that the predominant signals L L,,, R,, and R in both are identical in relative phase and amplitude to the corresponding signals at the input terminals of their respective encoders. but that their corresponding accompanying signals are equal and out of phase. For example. comparison of phasor groups 94 and 126 show that the L, signal in both is equal and in phase whereas the signals .7R,, and .7L,, in one are equal and out of phase with corresponding components in the other. Therefore. when the signal at terminal 82 of the HO. 3 decoder is added to the signal at terminal 114 of the decoder of FIG. 4, the desired L; signal will be augumented (doubled) and the undesired accompanying signals .7L,, and .7R,, will be cancelled. The same is true of the composite signals appearing at the output terminals 92, 88 and 84 of the decoder of FIG. 3 and the corresponding signals at the output terminals 120. 124 and 116 of the decoder of HO. 4. Thus. by matrixing the four original input signals with the differing encoders of FIGS. 1 and 2, and decoding the composite signals produced thereby with the differing decoders of FIGS. 3 and 4, respectively. and by combining the outputs of the two decoders in an additive mode. a set of four output signals can be obtained which in every way correspond to the original signals supplied to the encoders; i.e.. signals which are particularly independent of each other and contain the original directional infor mation. FIG. 5 illustrates how the outputs of the decodcrs of FIGS. 3 and 4 are added using the summing circuits 238 through 241 to obtain the four original individual audio signals. L L,,. R and R,, in fully discrete form.

As is evident from H65. 3 and 4, the complete decoding operation requires a total of 8 relatively expensive all-pass phase shifting networks, and it is an object of this invention to achieve the desired discrete outputs while reducing the necessary number of all-pass phase shift networks. FIG. 6 shows an embodiment of a decoder in accordance with the invention. The two composite signals, L and R and their conjugates, 1 and R are shown as being available at input terminals 301 through 304, respectively. Typically. these four signals will have been recovered from the principle and supplementary channels of a transmission or recording medium. The sum and difference of the signals L and L are formed using the summing circuits 311 and 313. Specifically. the output of summing circuit 311 is a first processed" signal at a terminal 321 and is seen to equal (.SL .5L while the output of summing circuit 313 is a second "processed" signal at a terminal 323 and is seen to equal (.71L .71L Similarly, the sum and difference of the signals R and R are formed using the summing circuits 312 and 314. The output of summing circuit 312 is a third processed signal at a terminal 322 and is seen to equal (.SR .5R while the output of summing circuit 314 is a fourth "processed" signal at a terminal 324 and is seen to equal (.71R .71R Due to the nature of the conjugate inputs, it is seen that the processed signals at terminals 321 and 322 are L, and R,. respectively. The processed signals at terminals 323 and 324 can be expressed as follows:

Terminal 323:

The signal at terminal 323, represented by the phasor group 325. is coupled through all-pass phase shift networks 331 and 332, the outputs ofthese phase shift networks being designated as auxiliary signals at the terminals 341 and 342. The signal at terminal 324, represented by the phasor group 326, is coupled through all pass phase shift networks 333 and 334, the outputs of these phase shift networks being designated as auxiliary" signals at the terminals 343 and 344. The phase shift networks 331 and 332 each introduce reference phase shift ll! whereas the phase shift networks 332 and 334 each introduce a phase shift of (111 As a result. the auxiliary signals at terminals 343 and 343 can be combined by summing circuit 351 to obtain the decoded original audio signal L,,. The signal at terminal 342, after the 90 relative phase shift introduced by network 332, can be represented as (L,, -jR,,). Thus the output of summing circuit 351 is which is the desired result. Similarly. a summing circuit 352 is used to obtain the decoded original audio signal R,,. The signal at terminal 344, after the 90 relative phase shift introduced by network 334. can be represented as (R +jL,,). so the output of the summing circuit 352 is h j lJ) h j II 0 The processed signals at terminals 321 and 322, which were seen before to be the individual audio signals L, and R,. respectively. are coupled through all pass phase shift networks 335 and 336, each of which introduces a reference phase shift. il. This is done so that the four decoded output signals at terminals 371, 372, 373 and 374 will have all experienced the same reference phase shift if: at all frequencies of interest and will retain their original phase relationships. Thus, the four desired discrete outputs are obtained using only six all-pass phase shifting networks. a reduction from the eight phase shifting networks employed in prior art techniques.

Another embodiment of the invention is illustrated in FIG. 7 and again includes four summing circuits (not shown) like the circuits 311 through 314 of FIG. 6 used to develop the processed signals at the terminals 321 through 324 of FIG. 6. These terminal designations are retained in FIG. 7. In FIG. 7, however. the number of necessary all-pass phase shift networks is reduced to four by compromising somewhat the integrity of the original phase relationships as between the individual audio signals. The processed signals at terminals 323 and 324 are phase shifted with respect to each other by 90. This is done by passing these signals through allpass phase shift networks 403 and 404 which respectively introduce phase shifts of ii; and (I1!- 90"). The outputs of networks 403 and 404 are represented by the phasor groups 405 and 406 which show that R,, components in the two signals are in phase whereas the L components in the two signals are in phase opposition. Thus. by adding and subtracting these two signals. using summing circuits 411 and 412, respectively. discrete decoded outputs R and L can be obtained at terminals 423 and 424, respectively The processed sig nals at terminals 312 and 322 are coupled through allpass phase shift networks 40l and 402 which each in troduce a phase shift of (I11 45] so that the decoded L, and R, signals are as shown at the terminals 42] and 422.

The decoded set of signals at terminals 421 through 424 are the discrete original individual audio signals. but they are seen to be relatively phase shifted with respect to their original relationship. Specifically. the front signals L,, R, retain their original phase relation ship while the side pairs L;. L and R;, R, are relatively shifted by 45. The back signals L,,. R,, are at 90 with respect to their original phase relationship. This is a reasonable compromise from a listeners standpoint considering the saving of two all-pass phase shift networks compared to the prior embodiment. ln addition to the true front signals, the side signals center left," and center right are formed between channels which are altered by only 45 with respect to each other. and the resultants are reasonably close to a true phase situation.

FIG. 8 shows another embodiment of the invention wherein only two all-pass phase shift networks need be employed The decoder of FIG. 8 is the same as the one shown in FIG. 7 except that the phase shift networks 401 and 402 are eliminated. This means that the processed signals at terminals 321 and 322, i.e.. L, and R;, are coupled to the terminals 42[ and 422 without experiencing reference phase shifts. These front signals will retain their original phase relationship, as is desirable. but the back signals, L and R,, will lose their original phase relationship with the front signals. Thus. in FIG. 8 the phasor representations for L and R,, are displaced by the reference phase angle whereas L and R, are not. (In previous embodiments the reference phase it: was omitted from phasor diagrams since all output signals had experienced this same reference phase shift]. The embodiment of FIG. 8 produces less desirable results than the prior embodiments but has the advantage of generating discrete outputs with only twopass phase shift networks, a considerable cost savings.

While the invention has been described with reference to particular embodiments, it will be appreciated that variations within the spirit and scope of the invention will readily occur to those skilled in the art. For example, while the illustrative embodiments show a particular type of SO encoded signal. the invention applies equally well, with only minor modification, to other encoded signals, such as the forward looking" type of SO signals described in the co-pending application Ser. No. 462.044 filed Apr. 18, l974 and entitled Compatible Four Channel Radio Broadcast and Receiving Systern."

I claim:

1. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form. a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth audio signals in sub-dominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals,

comprising:

a. first combining means which receives said first composite signal and said third composite signals and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals;

b. second combining means which receives said second composite signal and said fourth composite signals and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals;

. a first all-pass phase shifting circuit for phase shifting said first processed signal to obtain a first decoded signal;

d. a second all-pass phase shifting circuit for phase shifting said second processed signal to obtain a second decoded signal;

. a third all-pass phase shifting circuit for phase shifting said third processed signal by two different relative phase angles to obtain first and second auxiliary signals;

. a fourth all-pass phase shifting circuit for phase shifting said fourth processed signal by two different relative phase angles to obtain third and fourth auxiliary signals;

g. third means for combining said first and fourth auxiliary signals to obtain a third decoded signal; and

h. fourth means for combining said second and third auxiliary signals to obtain a fourth decoded signal.

2. An apparatus as defined by claim 1 wherein said third all-pass phase shifting circuit comprises a pair of all-pass phase shifters which receive said third processed signal in parallel and respectively introduce relative phase shifts differing by 3. An apparatus as defined by claim 2 wherein said fourth all-pass phase shifting circuit comprises a pair of all-pass phase shifters which receive said fourth processed signal in parallel and respectively introduce relative phase shifts differing by 90.

4. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form, a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth audio signals in sub-dominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals comprising:

a. first combining means which receives said first composite signal and said third composite signals and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals;

b. second combining means which receives said second composite signal and said fourth composite signal and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals;

c. first, second, third and fourth all-pass phase shifting circuits for phase shifting said first, second, third and fourth processed signals to obtain first, second, third and fourth phase shifted signals, said first phase shifted signal being a first decoded signal and said second phase shifted signal being a second decoded signal; and

d. third combining means for combining said third and fourth phase shifted signals to obtain a pair of outputs which are a function of the sum and difference of said third and fourth phase shifted signals, said outputs being said third and fourth decoded signals.

5. An apparatus as defined by claim 4 wherein said third and fourth phase shifting circuits introduce relative phase shifts differing by 90.

6. An apparatus as defined by claim 5 wherein said first and second phase shifting circuits each introduce relative phase shifts which differ by 45 from the relative phase shifts introduced by said third and fourth phase shifting circuits.

7. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form, a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth individual audio signals in subdominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals, comprising:

a. first combining means which receives said first composite signal and said third composite signal and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals, said first processed signal being a first decoded signal;

b. second combining means which receives said second composite signal and said fourth composite signal and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals, said third processed signal being a second decoded signal;

c. first and second all-pass phase shifting circuits for phase shifting said second and fourth processed signals to obtain first and second auxiliary signals; and

d. third combining means for combining said first and second auxiliary signals to obtain a pair of outputs which are a function of the sum and difference of said first and second auxiliary signals, said outputs being third and fourth decoded signals.

8. An apparatus as defined by claim 7 wherein said first and second phase shifting circuits introduce relative phase shifts differing by 

1. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form, a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth audio signals in sub-dominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals, comprising: a. first combining means which receives said first composite signal and said third composite signals and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals; b. second combining means which receives said second composite signal and said fourth composite signals and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals; c. a first all-pass phase shifting circuit for phase shifting said first processed signal to obtain a first decoded signal; d. a second all-pass phase shifting circuit for phase shifting said second processed signal to obtain a second decoded signal; e. a third all-pass phase shifting circuit for phase shifting said third processed signal by two different relative phase angles to obtain firSt and second auxiliary signals; f. a fourth all-pass phase shifting circuit for phase shifting said fourth processed signal by two different relative phase angles to obtain third and fourth auxiliary signals; g. third means for combining said first and fourth auxiliary signals to obtain a third decoded signal; and h. fourth means for combining said second and third auxiliary signals to obtain a fourth decoded signal.
 2. An apparatus as defined by claim 1 wherein said third all-pass phase shifting circuit comprises a pair of all-pass phase shifters which receive said third processed signal in parallel and respectively introduce relative phase shifts differing by 90*.
 3. An apparatus as defined by claim 2 wherein said fourth all-pass phase shifting circuit comprises a pair of all-pass phase shifters which receive said fourth processed signal in parallel and respectively introduce relative phase shifts differing by 90*.
 4. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form, a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth audio signals in sub-dominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals comprising: a. first combining means which receives said first composite signal and said third composite signals and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals; b. second combining means which receives said second composite signal and said fourth composite signal and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals; c. first, second, third and fourth all-pass phase shifting circuits for phase shifting said first, second, third and fourth processed signals to obtain first, second, third and fourth phase shifted signals, said first phase shifted signal being a first decoded signal and said second phase shifted signal being a second decoded signal; and d. third combining means for combining said third and fourth phase shifted signals to obtain a pair of outputs which are a function of the sum and difference of said third and fourth phase shifted signals, said outputs being said third and fourth decoded signals.
 5. An apparatus as defined by claim 4 wherein said third and fourth phase shifting circuits introduce relative phase shifts differing by 90*.
 6. An apparatus as defined by claim 5 wherein said first and second phase shifting circuits each introduce relative phase shifts which differ by 45* from the relative phase shifts introduced by said third and fourth phase shifting circuits.
 7. Apparatus for decoding four individual audio signals to the extent they are contained in four encoded composite signals to recover the four individual audio signals in substantially their original form, a first of the composite signals containing the first individual audio signal in a dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, a second of the composite signals containing the second individual audio signal in dominant proportion and the third and fourth individual audio signals in sub-dominant proportions and phase shifted with respect to each other, said third and fourth composite signals being respectively conjugates of said first and second composite signals, comprising: A. first combining means which receives said first composite signal and said third composite signal and generates first and second processed signals which are a function of the sum and difference of said first and third composite signals, said first processed signal being a first decoded signal; b. second combining means which receives said second composite signal and said fourth composite signal and generates third and fourth processed signals which are a function of the sum and difference of said second and fourth composite signals, said third processed signal being a second decoded signal; c. first and second all-pass phase shifting circuits for phase shifting said second and fourth processed signals to obtain first and second auxiliary signals; and d. third combining means for combining said first and second auxiliary signals to obtain a pair of outputs which are a function of the sum and difference of said first and second auxiliary signals, said outputs being third and fourth decoded signals.
 8. An apparatus as defined by claim 7 wherein said first and second phase shifting circuits introduce relative phase shifts differing by 90*. 